Field
The present invention relates to wind turbine blades.
Description of the Related Art
Wind turbines typically include one or more blades to capture the kinetic energy of the wind. During use, the blades are subject to various aerodynamic and inertial loads which typically occur in the edgewise direction and in the flapwise direction. Edgewise loads are those parallel to the chord of the blade, while flapwise loads are perpendicular to the edgewise direction. The direction of these loads can be seen in FIG. 1, in which “x” denotes the direction of edgewise loads and “y” denotes the direction of flapwise loads.
In order to increase the proportion of available wind energy captured by a particular wind turbine, it is known to increase the length of the blades to increase the swept area of the turbine. However, as each blade rotates, inertial and aerodynamic forces along the blade result in an edgewise loading. As the blade passes the tower until it reaches the azimuth, the trailing edge of the blade is compressed due to gravitational loading. Furthermore, in some cases where the rotor is accelerating, such as start up and grid loss, the trailing edges of the blades have additional compressive loading. This may cause buckling of the trailing edge, as shown in FIG. 2.
To prevent buckling of the trailing edge, it is known to add reinforcing material to the blade in order to increase its rigidity. For example, in large blades it is known to increase the thickness of the undercamber in order to increase the rigidity of the blade in the region of the trailing edge, as shown in FIG. 3.
However, this increases the thickness of the trailing edge, resulting in greater levels of noise, poor aerodynamic performance and parasitic mass. Further, this effect is exacerbated in longer blades (>45 m) since the linear velocity of a wind turbine blade is proportional to the rotor diameter and higher speeds create more noise.
It is also known to apply an adhesive reinforcing tape to the outer surface of the fairing in the region of the trailing edge, particularly in the case of blades having aerodynamic fairings made of fibre reinforced plastic. Since the rigidity of the blade is lowest at the trailing edge tip and the compressive loading is greatest at this point, the reinforcing tape is typically placed as close as possible to the tip of the trailing edge. However, this also results in an increase in the thickness of the trailing edge, resulting in greater levels of noise.
US Patent Application Publication No. 2010/0047070 discloses a blade with a sacrificial plastic element fixed into the trailing edge to prevent the otherwise periodical alternating vortex shedding from the trailing edge. Although this document expresses a preference for the plastic element to have a sharp edge, it does not specify any particular requirement for the shape of the plastic element. Some examples have a flexible strip on the top and bottom surfaces of the plastic element to act as lightning protection